1. Field of the Invention
The present invention relates to a hydraulic control system for an automatic transmission, and more particularly, to a hydraulic control system that can enhance the responsiveness of a pressure switch while preventing the pressure switch from on/off hunting due to hydraulic vibration.
2. Description of the Related Art
A hydraulic control system for an automatic transmission disclosed in Japanese Patent No. 2925505 includes a pressure switch that detects the hydraulic pressure produced in a hydraulic circuit. In this system, on/off hunting of the pressure switch occurs because of hydraulic vibration (turning-on and -off due to irregular vibration), and this reduces the durability of the pressure switch.
Accordingly, Japanese Unexamined Patent Application Publication No. 2002-89671 discloses that unstable hydraulic-pressure detection signals output from the pressure switch because of hydraulic vibration are removed by the timer setting. However, precise and stable hydraulic-pressure detection signals can be obtained only after hydraulic vibration subsides, and this increases the detection time.
In a control system disclosed in Japanese Unexamined Patent Application Publication No. 2003-97690, the line pressure is reduced and is then used as a supply pressure for a pressure switch, and a valve is provided between the supply pressure and the pressure switch to prevent the pressure switch from being affected by hydraulic vibration. In this structure, however, when the valve between the supply pressure and the pressure switch sticks, the pressure switch does not operate. Therefore, the system is not suitable for popular use, and the cost thereof increases.
FIG. 8 shows a known hydraulic control system 101 for an automatic transmission. In the hydraulic control system 101, an orifice 108 and an accumulator 102 for reducing hydraulic vibration are provided to prevent on/off hunting of a pressure switch 103. For example, a transmission with a variable line pressure disclosed in U.S. Pat. No. 6,471,613 adopts a pressure switch and an accumulator.
The pressure switch is used for fail-safe purpose, and therefore, is required to have high responsiveness. However, the responsiveness of the pressure switch is lowered by the orifice and the accumulator, and there is not sufficient time in the event of failure (during a non-operation state).